Instantiate pref controllers from xml if it's defined.

- If a <preference> tag also defines a controller, we will try to
  instantiate it before displaying the UI. The same logic is shared by
  BaseSearchIndexProvider so it also drives search suppression.

- If user also defines a list of controllers programatically, the
  programatically created ones takes precedence.

Bug: 73668763
Test: WIP
Change-Id: I7aecec270bcd3af261e012ef1f6995d2a523cfa1

tests/robotests/src/com/android/settings/enterprise/EnterprisePrivacySettingsTest.java

@@ -98,7 +98,7 @@ public final class EnterprisePrivacySettingsTest {
     @Test
     public void getSearchIndexProviderPreferenceControllers() throws Exception {
         final List<AbstractPreferenceController> controllers
-                = EnterprisePrivacySettings.SEARCH_INDEX_DATA_PROVIDER.getPreferenceControllers(
+                = EnterprisePrivacySettings.SEARCH_INDEX_DATA_PROVIDER.getAllPreferenceControllers(
                 ShadowApplication.getInstance().getApplicationContext());
         verifyPreferenceControllers(controllers);
     }